Google the wheel cylinder and master cylinder part #'s to find rebuild kits and/or possibly replacements. #'s are cast on them. I did the brakes on an old 60's IH dump truck last summer. NO wheel cylinders were available, but did find cup/spring/dust cap kits. Weren't cheap! Napa found the rubber hose from frame to rear axle after looking thru 50 pages of pictures in their books. You will have to have brake shoes relined and pray you don't need drums! I have a couple D500 axles that might have usable drums and other parts if you strike out elsewhere.

Plastic trucks suck...
Iron trucks rule...